5-hydroxytryptamine stimulation of phospholipase D activity in the rabbit isolated mesenteric artery

Structure and function of serotonin G protein-coupled receptors

Serotonin receptors are prevalent throughout the nervous system and the periphery, and remain one of the most lucrative and promising drug discovery targets for disorders ranging from migraine headaches to neuropsychiatric disorders such as schizophrenia and depression. There are 14 distinct serotonin receptors, of which 13 are G protein-coupled receptors (GPCRs), which are targets for approximately 40% of the approved medicines. Recent crystallographic and biochemical evidence has provided a converging understanding of the basic structure and functional mechanics of GPCR activation. Currently, two GPCR crystal structures exist for the serotonin family, the 5-HT1B and 5-HT2B receptor, with the antimigraine and valvulopathic drug ergotamine bound. The first serotonin crystal structures not only provide the first evidence of serotonin receptor topography but also provide mechanistic explanations into functional selectivity or biased agonism. This review will detail the findings of these crystal structures from a molecular and mutagenesis perspective for driving rational drug design for novel therapeutics incorporating biased signaling.

The TRPM8 channel forms a complex with the 5-HT(1B) receptor and phospholipase D that amplifies its reversal of pain hypersensitivity

Effective relief from chronic hypersensitive pain states remains an unmet need. Here we report the discovery that the TRPM8 ion channel, co-operating with the 5-HT(1B) receptor (5-HT(1B)R) in a subset of sensory afferents, exerts an influence at the spinal cord level to suppress central hypersensitivity in pain processing throughout the central nervous system. Using cell line models, ex vivo rat neural tissue and in vivo pain models, we assessed functional Ca(2+) fluorometric responses, protein:protein interactions, immuno-localisation and reflex pain behaviours, with pharmacological and molecular interventions. We report 5-HT(1B)R expression in many TRPM8-containing afferents and direct interaction of these proteins in a novel multi-protein signalling complex, which includes phospholipase D1 (PLD1). We provide evidence that the 5-HT(1B)R activates PLD1 to subsequently activate PIP 5-kinase and generate PIP2, an allosteric enhancer of TRPM8, achieving a several-fold increase in potency of TRPM8 activation. The enhanced activation responses of synaptoneurosomes prepared from spinal cord and cortical regions of animals with a chronic inflammatory pain state are inhibited by TRPM8 activators that were applied in vivo topically to the skin, an effect potentiated by co-administered 5-HT(1B)R agonists and attenuated by 5-HT(1B)R antagonists, while 5-HT(1B)R agents alone had no detectable effect. Corresponding results are seen when assessing reflex behaviours in inflammatory and neuropathic pain models. Control experiments with alternative receptor/TRP channel combinations reveal no such synergy. Identification of this novel receptor/effector/channel complex and its impact on nociceptive processing give new insights into possible strategies for enhanced analgesia in chronic pain.

Co-expression of serotonin 5-HT(1B) and 5-HT(4) receptors in p11 containing cells in cerebral cortex, hippocampus, caudate-putamen and cerebellum

p11 is an adaptor protein which binds to serotonin 5-HT(1B) receptors and 5-HT(4) receptors and regulates their localization at the cell surface. In the present study, we examined to what extent p11 containing neurons co-expressed 5-HT(1B)R and/or 5-HT(4)R in cerebral cortex, hippocampus, cerebellum and caudate-putamen. A triple-labeling immunohistochemical approach was taken using antibodies to detect native p11 and 5-HT(1B)R combined with visualization of EGFP driven under the 5-HT(4)R promoter in BAC-transgenic mice. In the caudate-putamen, the hippocampal pyramidal cell layer of CA1 and the hippocampal granule cell layer of dentate gyrus, most p11 containing cells co-expressed both 5-HT(1B)R and 5-HT(4)R. In the cingulate cortex, stratum radiatum/oriens of CA1, hilus of the dentate gyrus and cerebellar cortex, many cells co-expressed p11 and 5-HT(1B)R, but not 5-HT(4)R. In the studied brain regions, few cells solely expressed p11 without any significant expression of 5-HT(1B)R or 5-HT(4)R. It can be concluded that p11 is anatomically positioned to modulate serotonin neurotransmission, via 5-HT(1B)R and 5-HT(4)R, in brain regions important for emotionality, cognition and locomotion.

Phospholipase D signaling in serotonin-induced mitogenesis of pulmonary artery smooth muscle cells

We have previously reported the participation of mitogen-activated protein, Rho, and phosphoinositide-3 (PI3) kinases in separate pathways in serotonin (5-HT)-induced proliferation of pulmonary artery smooth muscle cells (SMCs). In this study, we investigated the possible participation of phospholipase D (PLD) and phosphatidic acid (PA) in this growth process. 5-HT stimulated a time-dependent increase in [(3)H]phosphatidylbutanol and PA generation. Exposure of SMCs to 1-butanol or overexpression of an inactive mutant of human PLD1R898R blocked 5-HT-induced proliferation. Furthermore, 1-butanol inhibited 5-HT activation of S6K1 and S6 protein, downstream effectors of mammalian target of rapamycin (mTOR), by 80 and 72%, respectively, and partially blocked activation of extracellular signal-regulated kinase (ERK) by 30% but had no effect on other associated signaling pathways. Exogenous PA caused cellular proliferation and revitalized cyclin D1 expression by 5-HT of the 1-butanol-treated cells. PA also reproduced activations by 5-HT of mTOR, S6K1, and ERK. Transfection with inactive human PLD1 reduced 5-HT-induced activation of S6K1 by approximately 50%. Inhibition of 5-HT receptor 2A (R 2A) with ketaserin blocked PLD activation by 5-HT. Inhibition with PI3-kinase inhibitor failed to block either activation of PLD by 5-HT or PA-dependent S6K1 phosphorylation. Taken together, these results indicate that ligation of the 5-HTR 2A by 5-HT initiates PLD activation in SMCs, and that its product, PA, is an early signaling molecule in 5-HT-induced pulmonary artery SMC proliferation. Signaling by PA produces its downstream effects primarily through the mTOR/S6K1 pathway and to a lesser extent through the ERK pathway. Hydrolysis of cell membrane lipid may be important in vascular effects of 5-HT.

Comparison of signalling mechanisms involved in rat mesenteric microvessel contraction by noradrenaline and sphingosylphosphorylcholine

1 We have compared the signalling mechanisms involved in the pertussis toxin-sensitive and -insensitive contraction of rat isolated mesenteric microvessels elicited by sphingosylphosphorylcholine (SPC) and noradrenaline (NA), respectively. 2 The phospholipase D inhibitor butan-1-ol (0.3%), the store-operated Ca(2+) channel inhibitor SK>F 96,365 (10 microM), the tyrosine kinase inhibitor genistein (10 microM), and the src inhibitor PP2 (10 microM) as well as the negative controls (0.3% butan-2-ol and 10 microM diadzein and PP3) had only little effect against either agonist. 3 Inhibitors of phosphatidylinositol-3-kinase (wortmannin and LY 294,002, 10 microM each) or of mitogen-activated protein kinase kinase (PD 98,059 and U 126, 10 microM each) did not consistently attenuate NA- and SPC-induced contraction as compared to their vehicles or negative controls (LY 303,511 or U 124). 4 The phospholipase C inhibitor U 73,122 (10 microM) markedly inhibited the SPC- and NA-induced contraction (70% and 88% inhibition of the response to the highest NA and SPC concentration, respectively), whereas its negative control U 73,343 (10 microM) caused only less than 30% inhibition. 5 The rho-kinase inhibitors Y 27,632 (10 microM) and fasudil (30 microM) caused a rightward-shift of the NA concentration-response curve by 0.7-0.8 log units and reduced the response to 10 microM SPC by 88% and 83%, respectively. 6 These data suggest that SPC and NA, while acting on different receptors coupling to different G-protein classes, elicit contraction of rat mesenteric microvessels by similar signalling pathways including phospholipase C and rho-kinase.

Multiplicity of mechanisms of serotonin receptor signal transduction

The serotonin (5-hydroxytryptamine, 5-HT) receptors have been divided into 7 subfamilies by convention, 6 of which include 13 different genes for G-protein-coupled receptors. Those subfamilies have been characterized by overlapping pharmacological properties, amino acid sequences, gene organization, and second messenger coupling pathways. Post-genomic modifications, such as alternative mRNA splicing or mRNA editing, creates at least 20 more G-protein-coupled 5-HT receptors, such that there are at least 30 distinct 5-HT receptors that signal through G-proteins. This review will focus on what is known about the signaling linkages of the G-protein-linked 5-HT receptors, and will highlight some fascinating new insights into 5-HT receptor signaling.

Cardiovascular effects of SL65.0472, a 5-HT receptor antagonist

In this study, we describe the cardiovascular effects of SL65.0472 (7-fluoro-2-oxo-4-[2-[4-(thieno[3,2-c] pyridin-4-yl) piperazin-l-yl] ethyl]-1, 2-dihydroquinoline-1-acetamide), a novel 5-hydroxytryptamine (5-HT) receptor antagonist developed for the treatment of cardiovascular disease, in several in vivo models. The haemodynamic profile of SL65.0472 was evaluated in anaesthetised dogs. Following i.v. bolus doses of 0.03 mg/kg i.v. and 0.3 mg/kg, no significant changes in cardiac output, contractility or rate, systemic and pulmonary pressures, regional blood flows and vascular resistances or electrocardiogram were noted. After 1 mg/kg i.v. SL65.0472 significantly reduced arterial blood pressure. In conscious spontaneously hypertensive rats administration of SL65.0472 0.5 mg/kg p.o. had no effect on mean arterial blood pressure or heart rate. Vasoconstriction produced by 5-HT results primarily from the stimulation of two receptor subtypes, 5-HT(1B) and 5-HT(2A) receptors. In anaesthetised dogs SL65.0472 antagonised sumatriptan-induced decreases in saphenous vein diameter (5-HT(1B)-receptor mediated) with an ID(50) of 10.1 microg/kg i.v. (95% c.l. 8.3-12.4). In anaesthetised pithed rats SL65.0472 inhibited 5-HT pressor responses (5HT(2A)-receptor mediated) with ID(50) values of 1.38 microg/kg i.v. (95% c.l. 1.15-1.64) and 31.1 microg/kg p.o. (95% c.l. 22.6-42.6). The duration of the 5-HT(2A)-receptor antagonist effect of SL65.0472 following oral administration was evaluated in conscious rats. SL65.0472 (0.1 mg/kg p.o.) markedly inhibited 5-HT pressor responses 1 and 6 h after administration. Therefore, in vivo, SL65.0472 potently antagonises vasoconstriction mediated by 5-HT(1B) and 5-HT(2A) receptors but has minimal direct haemodynamic effects.

Effects of SL 65.0472, a novel 5-HT receptor antagonist, on 5-HT receptor mediated vascular contraction

5-hydroxytryptamine (5-HT) contracts vascular smooth muscle and pharmacological and molecular biological data suggest that these effects are mediated primarily by stimulation of 5-HT(1B) and 5-HT(2A) receptor subtypes. We have studied the properties of 7-fluoro-2-oxo-4-[2-[4-(thieno[3,2-c] pyridin-4-yl) piperazin-1-yl] ethyl]-1,2-dihydroquinoline-1-acetamide (SL 65.0472 ), a novel 5-HT receptor antagonist, in isolated vascular preparations contracted by 5-HT or sumatriptan. In canine isolated saphenous vein strips (putatively 5-HT(1B)-mediated contraction), SL 65.0472 antagonised sumatriptan-induced contractions in a competitive manner (pA(2) 8. 17+/-0.36). 5-HT contracts rabbit aorta by stimulation of 5-HT(2A) receptors. SL 65.0472 displaced the 5-HT concentration response curve in rabbit aorta rightwards with a significant reduction in maximum. The apparent pK(B) value was 8.58+/-0.18. 5-HT-induced contractions of human coronary arteries are mediated by a mixed population of 5-HT(1B) and 5-HT(2A) receptors. SL 65.0472 produced rightward parallel shifts of the 5-HT concentration response curves in all tissues studied (pA(2) 8.8+/-0.14, n=7). In conclusion, SL 65. 0472 is a potent antagonist of vascular smooth muscle contraction in vitro mediated by 5-HT receptor stimulation.

The involvement of intracellular Ca(2+) in 5-HT(1B/1D) receptor-mediated contraction of the rabbit isolated renal artery

5-Hydroxytryptamine(1B/1D) (5-HT(1B/1D)) receptor coupling to contraction was investigated in endothelium-denuded rabbit isolated renal arteries, by simultaneously measuring tension and intracellular [Ca(2+)], and tension in permeabilized smooth muscle cells. In intact arterial segments, 1 nM - 10 microM 5-HT failed to induce contraction or increase the fura-2 fluorescence ratio (in the presence of 1 microM ketanserin and prazosin to block 5-HT(2) and alpha(1)-adrenergic receptors, respectively). However, in vessels pre-exposed to either 20 mM K(+) or 30 nM U46619, 5-HT stimulated concentration-dependent increases in both tension and intracellular [Ca(2+)]. 1 nM - 10 microM U46619 induced concentration-dependent contractions. In the presence of nifedipine (0.3 and 1 microM) the maximal contraction to U46619 (10 microM) was reduced by around 70%. The residual contraction was abolished by the putative receptor operated channel inhibitor, SKF 96365 (2 microM). With 0.3 microM nifedipine present, 100 nM U46619 evoked similar contraction to 30 nM U46619 in the absence of nifedipine, but contraction to 5-HT (1 nM - 10 microM) was abolished. In permeabilized arterial segments, 10 mM caffeine, 1 microM IP(3) or 100 microM phenylephrine, each evoked transient contractions by releasing Ca(2+) from intracellular stores, whereas 5-HT had no effect. In intact arterial segments pre-stimulated with 20 mM K(+), 5-HT-evoked contractions were unaffected by 1 microM thapsigargin, which inhibits sarco- and endoplasmic reticulum calcium-ATPases. In vessels permeabilized with alpha-toxin and then pre-contracted with Ca(2+) and GTP, 5-HT evoked further contraction, reflecting increased myofilament Ca(2+)-sensitivity. Contraction linked to 5-HT(1B/1D) receptor stimulation in the rabbit renal artery can be explained by an influx of external Ca(2+) through voltage-dependent Ca(2+) channels and sensitization of the contractile myofilaments to existing levels of Ca(2+), with no release of Ca(2+) from intracellular stores.